Things to look for:
Have them pull an Oasis through Ford and see if it has had any issues, specifically replaced coils/plugs/cat convertor. That is a sign of the condensation issue mentioned above. It was actually pretty limited/low percentage of trucks it shows up on. Most people drill a tiny weep hole in the bottom of the intercooler and never experience the issue again. I never had the issue, but I did drill the weep hole in my for safe measure. 31k on my 2011 model, and 8k of those pulling my TT with 100% dependability.
You also should shoot for a 3:55 gear as best for towing. 3:31 prob do pretty decent, but 355 and higher is best.
Open the drivers door and look for the yellow sticker. A payload/cargo capacity of 1500# or so means you have a decent ring and pinion and a truck that isn't weighed down with a bunch of options. There are platinum F150s out there with only 1100 cargo capacity, which can be limiting when pulling a trailer.